Musical key control



Oct. 26, 1948. KOEHL 2,452,307

IUSICAL KEY CONTROL Filed larch 20, 1946 2 Shuts-Sheet 1 Patented Oct. 26, 1948 MUSICAL KEY CONTROL James'A. koehl, Evanston, Ill., assignor to Central Commercial Company, Chicago, 111., a corporation of Illinois 1 Application March 20, 1946, Serial No. 655,649

7 Claims. (Cl. 201-55) 1 This invention relates to electric switches. The herein disclosed electricswitch is intended primarily for use in connection with the playing keys of electrical musical instruments employing generators 01' tone signal voltages adapted on depression of selected playing-keys to impress signal voltages on the input of an amplifier, the output circuit of which is connected to a loud speaker for translation of the amplifiedsignals into audible sound for musical expression.

I An object of the invention is the provision 01 an electric switch mr varying the amplitude of signal voltage in the path connecting a given generator to theai'orementioned input circuit of said ampliiler and preventing generation at the loud speaker of noise commonly referred to as "key thump, the same being the resultant of sudden electron surges when close circuiting said generators with said ampliiier.

An important object of the invention is the provision of a self cleaning switch, the operation of which is dependable throughout long periods of time and is simple construction, inexpensive, compact and of such form and design that it can be embodied in a musical instrument and readily associated with a playing-key for actuation thereof by the key. 1

Another object is the provision of resistive means adapted to be connected in shunt across a signal transmission path andis variable in a manner insuring gradual increase and decrease in amplitude of signal voltage in response to motion of a playing-key.

Other and further objects and advantages of my invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure l is a diagram illustrating the broad form of my switch showing same connected in shunt across a signal voltage transmitting path; Figure 2 is aside elevation of the switch showing same in coaction with the playing-key of a musical instrument for actuation oi the switch in response to motion of said key, said switch being in an oil position; Figure-3 is a view similar to Figure 2 showing the switch in a partly on positionfFigure 4 is a transverse section'taken on line 4-4 of Figure 2; Figure 5 is a'view similar to Figure 3 showing a slightly modified form of my invention, and Figure 6 is a plan view of the mounting member for the fixed contact elements 01' the switch. Figure 7 is a diagram depicting the function of my variable resistance in shunt across a resistive signal voltage transmission channel.

Referring to Figure l, i is a generator or source of signal voltage, such as an electron discharge device or any suitable well known type designed to produce output signal voltage of given tone frequency. 2 denotes an amplifier comprising a space discharge device such as a vacuum tube of any well known type. 3 is a signal voltage transmission channel connecting the output circuit of signal source I to the input circuit 01. amplifier 2, the same having resistance R1 in series therewith.

4 is a metallic strip connected to ground 5 and provided with a longitudinal series of spaced apart contact elements 6, Ga, 6b and 80, adapted, when the strip is in the o position shown at Figure 2, concurrently to engage respective, longitudinally spaced apart fixed contact elements I, la, lb and 10, connected together by series resistances 8, 8a and 8b. Elements 6c and 1c are always engaged with each other regardless of whether the switch is in an on or oil position. A lead wire 9 connects the switch in shunt across said channel 3, and by virtue thereof, generator I is completely grounded through said strip 4, thereby preventing transmission of signal voltage to said amplifier. Strip 4 is pivoted at it and said fixed contact elements 1, 1c and lb are so formed and related to each other and to said strip 4, as to sequentially disengage coactive elements t, to. and to when moving said strip in an. on" direction. When the strip is in motion in an 011" direction said contact elements 6, 6a and 8b will reengage said contact elements i, la and lb, the order of their engagement, however, being the reverse of that when moving said strip in an on direction, i. e. contact element 6b will be the first to engage its contact element 1b.

With the strip in the full "oh position, signal voltage from generator i is shunted to ground through element 6 oisaid strip. Resistances 8, 8a and 8b are connected in series with ground 5 and are of predetermined values such that the total resistance across path 3 is a predetermined sum of such resistances that are effectively in the shunt path at any instant. Tracing gradual motion of strip 4 in an "on direction, it follows that the total effective resistance added in the shunt path increases continuously and progressively with successive disengagement of contact elements 6, Ba and 61: from coacting fixed contact elements I, la. and lb. As previously stated, when said strip 4 is in a full "oiP position no signal voltage passes to amplifier 2. .When contact element 8 on strip 4 disengages contact element '1, resistance 8b is introduced in the shunt path, at which time, contact elements Ba and la are still engaged with each other. Upon disengagement of element to from element la, resistance 8a is also introduced in the shunt path and added to resistance 8b, the path to ground then being through said resistances and strip 4, the contact element 8b of which is still engaged with contact element lb. when strip 4 has been moved to a full "on" position, element Sb will have disengaged element lb, but element to will have maintained engagement with element lo and the path to ground will then be through resistances 8b, 8c and 8 and through elements'tc and Tic and strip Q. It will be pointed out hereinafter that contact elements i, la, lb and la are of graduated lengths, such that when element t is fully disengaged from element l, element to. is still engaged with element la. This action of timing engagement and disengagement of the coactive contact elements follows in like manner throughout the entire series of contact elements, contact element 80, however, remaining engaged with contact element ic regardless of the condition of the switch at anyinstant. It follows that by this serial order of varying resistance to the flow of signal voltage to ground, the amplitude of voltage being transmitted to the amplifier gradually increases from zero to that level which is the resultant of the total resistance to ground at any instant. Assuming that the amplifier is connected to an electroacoustical translating device, such as a loud speaker, the volume of sound at the speaker will increase as a function of a variable resistance in the shunt path. In this manner, key thump noise which otherwise would be experienced is effectively suppressed. Movement of strip 4 in an opposite direction sequentially decreases the resistance in the shunt path until all fixed contact elements have been grounded by a full off position of said strip.

In carrying the invention into practice use is made of a strip ii of Bakelite or other well known insulating material of suitable length and width. Vertical equi-distantly spaced apart holes H are punched in said strip ii and passing through same and upset at the opposite sides thereof are eyelets i3. Firmly secured in each of said eyelet is a metallic plug ll having an exposed tapered end IS. The holes are arranged in a longitudinal series and as shown, the plugs M are respectively of varying lengths such that a straight line passing across the tapered tips thereof is slightly inclined relativeto the plane of said strip I I, the longest plug being disposed adjacent to the front end I 8 of said strip from which point the remaining plugs of the series progressively diminish in length as shown at Figures 2 and 3 of the drawings. Said front end of said strip is formed with a vertical slot H for a purpose to appear presently. The upper face of the strip II is coated with carbon i8, the method of coating being such that a carbon line connects one hole l2. with the next adjacent hole as shown at Figure 6.

At the back end of strip i I is a stud l 8 to which is pivoted at l9 a flat, thin strip 20 of conductive material such as phosphor-bronze. The strip 20 functions in the manner of the movable contact element of a knife switch. The front end of said strip is bent angularly and freely passes through slot l1 and is formed beneath said slot to provide a foot piece 2i, which is normally urged in a downward direction by a spring extension 22 which is normally backed up and tensioned by a stop pin 23 on said stud l 8.

Said strip 20 is provided with effective contact elements 24, each of which is substantia y plike and provided with outwardly flared teal ends 25, 25. These elements are mounted in a longitudinal series and equidistantly spaced apart the same as studs 66 with which they respectively coact, it being understood that for each individual stud there is one of said clips, and that from the construction of these coactive contact elements wiping engagement of one element against a coactive element is assured during each operation of the switch, thereby rendering said elements self cleaning and insuring good electrical connections.

In Figures 2 and 3 of the drawings, the switch is shown in association with a playing-key K of an electrical musical instrument, such as an electric organ wherein for each individual playingkey in the instrument's gamut there is a generator similar to generator i first above referred to, each generator providing a waveform of signal voltage in its output circuit, the frequency of which is the same as the vibration frequency of the note of a musical scale with which said key is associated. For each playing-key there will be one of said switches, the foot piece ii of which is coactive with the back end of said key, whereby, when depressing said key, swinging motion is imparted to said strip 265. In this manner, said strip is moved against the normal urging tendency of the springy end 22 of said' strip. The extent of motion of the strip in an "on direction is determined by the extent of depression of said playing-key. The limit of motion of said strip is determined by the depth of depression of the playing-key. The playing-key and the switch are coordinated and function in such manner that when the playing-key is fully depressed, the last pair to the left of coacting contacts it and it will remain engaged with each other. A bracket B serves to mount the switch on the keyboard bedplank P.

The carbon coatings on the upper face of strip ii may be taken to be the resistances 8, 8a and 821 shown at Figure 1. While I prefer the use of carbon coatings, any equivalent resistance can be substituted therefor. The coatings are so formed that the value of the resistance is determined by the extent of the coating at any predetermined between 3a and do will be If R2 is a fixed resistance and RI a resistance which is variable from zero to some finite value,

V then becomes a function of Hi. We will assume that BI is connected by some mechanical arrangement such as the aforementioned switch, whereby gradual descending motion of the playing-key radually increases the resistance of RI from zero when the key is fully elevated and is at rest to some finite value when the key is depressed. Thus, it is apparent that by proper selection of the taper of resistance RI or by proper mechanical linkage between the key and RI or by a combination of the two, that any growth of voltage can be attained for a linear movement of the key and that the exponential rise of the resultant curve may be attained, thereby suppressing the objectionable thump" or "pop that otherwise would be experienced.

I am conversant with devices of the prior art 2,4saso7 wherein variations oi voltage is controlled by the rate at which a condenser charges and discharges. Such circuits include resistances connected with such condenser. Such devices are time delay means for controlling the rate of growth and decay oi musical tones. Once a key is moved for activation of such device, the player has no control of the situation and the voltage rises from zero to some peak value as a function oi the condenser charged and discharged through resistances. In the instant example of my invention, the depth of depression of the playing-key determines the volume 01' sound at a loud speaker, such that when the key is only partly depressed the volume of sound at the speaker will be less relative to the volume of sound whenever the key is depressed a further distance from a normal elevation. A switch such as the one disclosed herein is considerably more simple than one wherein the rate at which voltage rises is determined by the time to charge and discharge a condenser upon actuation of a playing-key. I am also aware of the fact that according to the functional action of devices of the prior art, variable resistances have been employed for determining transmission of voltage in a transmission channel. I know of no prior device embodying a series of pairs of longitudinally spaced apart coactive contact elements in which each pair is connected by a resistance of given value to elements of a next adjacent pair of elements in such series and are adapted to be actuated according to a fixed or given sequence. Neither do I know of any prior device wherein the respective coactive contact elements function in sequence and determine the amount oi resistance in shunt across a transmission channel and are so formed and designed that a succeeding contact element does not entirely break connection with its coacting contact as long as there is any coactive engagement of the elements of a preceding pair of elements. In this manner, there is no break or interruption in the flow of voltage during motion of such element as the strip 2t) embodied in my switch.

In Figure 5, the switch element 20, instead oi having a U-shaped springy clip 24 for and coactive with each fixed contact element M, is formed with a downwardly flared integral lip 25 which has wiping engagement therewith so that the eiTective surfaces of these elements are selfcleanlng and insure good electrical contact. The,

lips 1H5 are so formed that the alternate ones thereof preferably extend from one side of strip 28, whereas the intermediate lips extend from the opposite side of said strip.

As the playing-key of an electric organ or like instrument is always limited in its motion in a playing direction, it suflices to say that when the switch is mounted as herein shown and described, swinging motion of strip 20 in an "on direction is limited in such manner that clip 24 which coacts with the last fixed contact at the left, never entirely leaves said element. In this manner the resistive shunt path to ground is never completely broken.

What I claim as my invention is:

1. An electric switch comprising a variable resistance adapted to be connected in a shunt path across a signal transmission channel; a plurality of fixed contact elements electrically insulated from each other and connected with said resistance in such manner that each individual contact element denotes a predetermined value 0! resistance in said shunt path; a rocker of conductive material a plurality of movably mounted contact elements mounted on said rocker and disposed for electrical coaction with respective ones of said fixed contact elements; said rocker having a sprlngy portion tending to urge to a position of electrical engagement the second named contact elements with their respective fixed contact elements a common ground for said movably mounted elements and means actuable from a control key (or sequentially engaging said movably mounted elements with and similarly, sequentially disengaging same from said fixed elements.

2. An electric switch comprising a rectilinear conductive strip mounted for movement in an arc of a circle and provided with a longitudinal series of spaced apart contact elements and a springy portion at one of its ends, a ground connection with which said springy portion has constant engagement; a longitudinal series of spaced apart, fixed contact elements electrically insulated from each other and, disposed for coaction with respective contact elements on said conductive strip; series resistances connecting said fixed contact elements with each other and including 'a lead wire for connecting said conductive strip in shunt across a signal transmission path; and means actuable from a control key for moving said conductive strip, sequentially to disengage the contact elements on said strip from their coactive fixed contact elements during motion of said strip in one direction and similarly, sequentially to reengage the contact elements on said strip with their coactive fixed elements in consequence of movement of said strip in an opposite direction.

3. .An electric switch comprising a rectilinear conductive strip mounted for movement in an arc of a circle and provided with a lon itudinal series of spaced apart contact elements; a longitudinal series of spaced apart, fixed contact elements electrically insulated from each other, and disposed for coaction with respective contact elements on said conductive strip; series resistances connecting said fixed contact elements with each other and including a lead wire for connecting said conductive strip in shunt across a signal transmission path; and means actuable from a control key for moving said conductive strip, sequentially to disengage the contact elements on said strip from their coactive fixed contact elements during motion of said strip in one direction and similarly, sequentially to reengage the contact elements on said strip with their coactive fixed elements in consequence of movement of said strip in an opposite direction; and means forming a springy continuation of said conductive strip for urging said strip normally in a direction to veiiect and maintain engagement of said elements on said strip with their coactive fixed contact elements.

4. An electric switch comprising a. longitudinal series of contact elements electrically insulated from each other, a second longitudinal series of contact elements; a common mounting of conductive material for said second series of contact elements and formed at one end with an actuating foot and at its opposite end with a springy extension; means coactive with said springy extension for urging said common mounting in a direction eflecting engagement of the second named contact elements with said first named contact elements and a ground connection with which the springy extension has constant engagement.

5. An electric switch comprising a longitudinal series of spaced apart contact elements electrically insulated from each other; a second longitudinal series of spaced apart contact elements;

a common mounting of conductive material for said second named contact elements; means supporting said common mounting for pivotal movement and for yieldingly urging same in a direction to efl'ect coactive engagement of the second named elements with respective ones of the first named elements and for providing a constant ground for said common mounting, said first named elements being formed and proportioned to cause coactive engagement and disengagement of the first and second named elements in accordance with a given serial order.

6. An electric switch comprising a set of longttudinally spaced apart contact elements electrically insulated from each other mine a resistance, a terminal of which is adapted to be connected across a signal transmission channel; a metallic strip forming a constant ground terminal for said resistance and supported for limited pivotal motion from a position of concurrent engagement of the strip with all oi said contact elements to a position, the attainment of which is preceded by serial disengagement of the strip from all but one of said elements.

7. An electric switch tor use with a musical instruent 1 emnlo a frequency generation source and a playing key for contro a; distribution or the output of the generator to a load circuit, said switch comprising a rocker of con ductive material provided with a terminal for engaging a key, and a sprinsy terminal for urg-= ing the first named terminal against the key to hold the latter in an elevated position, a groundlug connection with which the spri terminal h constant engagement, and variable resistive means controlled by said rocker and coactive with said generator and said load circuit to were the resistance therein as a function of the motion oi said key.

JAhmS A. KO.

RWERENQES The following references are of record the file of this patent:

STATES PATENTS 

